Discriminator circuits



M. G. CROSBY 2,374,734

ATTORN EY May l, 1945 M. G. CROSBY* DISCRIMINATOR CIRCUIT Filed Feb. 5, 1943 2 sheets-sheet 2 T1 :l E

9 2 P, f Il E 3E l l ,a vd" 14 L5, 1 I ijf/tg; (HOA/f 5l JZ T L i f/f l f A ATTQRNEY Patented May 1, 1945 DisoRnvuNAToa CIRCUITS Murray G. Crosby, Riverhead, N. Y., assignor to Radio .Gorporationof America, a corporationy -of Delawarey Application February 5, 1943, serial Nb. 474,301A

r s claims. (o1. 250440) My lpresent invention relates to discriminator circuits, and more particularly' to novel 'and improved forms of discriminator circu'its"particu` larly adapted for automatic frequency fcontrol (hereinafter referred to as A LIFTC.,A l 1 One of themain objects of myA present invention is to provide a discriminator circuit, adapted to feed its output energy to a differential detector, which functions in themanner of animpedance inversion network. j. `j

Another important object of this invention `is to provide a circuit capable of translating free quencydeviations'from a predetermined reference frequency into predetermined impedance changes thereby to provide resultant output energy whose 4amplitude frequency deviations. Y y Y Another object of my invention is to provide a pair of rectiiers in polarity opposition, and the rectiers having respective input impedances Whose respective impedance-frequency charac# teristics are of complementary form.4 v Y Still-another object of my present invention is to provide an AFC discriminatori-rectifier ,system of the type wherein opposed're'ctifiers have'sparate `impedance input circuitsr of complementary characteristics, and both input circuit-.shaving a common middle frequency which 4is ,substantially that of the mean frequency offappliedhghfrequencyenergy.

Yet other objects of my invention are t'oirn` prove generally the simplicity and efficiency of A FC discriminator-rectifier circuitsjand4 more especially to provide such circuits in-anf economical and readily-manufacturablemanner.

The novel f eatureswhich I believe to be characteristic of my invention are set'forth with par- `ticularity inthe appended claims; the invention itself, however, as to bothv itsorganization and method of operation will best be understood by reference to the following' description, taken in connection with the drawings in which I have indcated'.diagrammatically several circuitV organizations whereby my Vinvention may be carried into effect.

In the drawings:

, Fig. 1 shows a circuit embodying the invention,

is afunction of the said Fig. 2 graphically illustrates the functioning of I quency signal energyl to the discriminator'circuit. The circuits preceding the transformer l may be the customary networks of a superheterodynereceiver. `Numeral 2 denotes an amplier'ofjsig# nal energy whose resonant output circuit includes the primarywinding 3 of the transformer I. If the receiver is' a superheterodyne, then am# pliiier 2 may amplify intermediate frequency (I. F.) energy in the kilocycle (kc.) or megacycle (mc.) ranges. Those'skilled in the artare' fully acquainted with the general manner of providing AFC for a superheterodyne receiver.' The AFC voltage output of the opposed rectiers will Abe applied to any desired device for controlling the frequency of the local oscillator which feeds its oscillations to the first detector. The Vamplifier 2 could feed its I. F. output energy toa parallel path to a second 'detector (not shown),vor' the audio output could be derivedfrom the opposed rectiiiers per se.

The present invention `ist not limited to use in receivers, since it may be employed in transmitters, or in any oscillatory system, forthe control of the frequency of oscillation. In suchfcasethe oscillatory energy to be controlled is fed to 'amplier 2, and the AFC Voltage is employed'to regulate the-frequency control device of thefoscillator. See, for example, my U. S. Patent No.

2,279,659, granted April 14, 1942. Thesearefall utilizations which are Well known to those skilled in the art, and need not be 'describedanyfurthe;1

herein. Y The condenser 4, in shunt across' primary winding 3, tunes the primary circuit tothe op'- the cross-over frequency Fc when condenser C is shorted. C is made equal to C` in Value. The value of Fe in the case of a superheterodyne receiver will be the operating I. F. Valuep'In .the

case of a transmitter master oscillator, orany oscillator in general whose 'frequency is to beregulated, the cross-over frequency will be the predetermined oscillator frequency.v At that crossover frequency F@ the impedances' presented' toi the opposed rectiflers 5 and 'l Will be equal.

The rectiers 6 and 1 may be diodes. "Thein vention is not limited to diodes, as other typeso'fv rectiers may be used.' The'anodev 8 of diode 6 is connected by blocking', or'isolation', ycondenser'S to the junction of coil 5 and `condenser C."The

spective high frequency current by-pass condenser. The cathode of diode 'I is grounded, while AFC voltage is taken off from the cathode end ofv resistor I2. The AFC voltage is appliedv to a control device for an oscillator.

llllach rectifier is connected across a respective input impedance element C or P-C. Considering Fig. 2, the-impedance change with frequency variation across the input impedance element of rectifier 6 is depicted by curve C. It will be noted that over the linear portion of characteristic C, between the peaks thereof, the impedance rises with frequency. The impedance-frequency characteristic P across the input element P has a falling course over the linear portion thereof. The series resonant crystal frequency, or peak frequency, is on one side of Fc; the dip, or antiresonant, crystal frequency is on the opposite side of Fc. The impedance-frequency characteristics P and C are complementary. The discrimination will occur by virtue of the fact that complementary impedance is presented to each rectifier.

As is seen from Fig. 2, the impedances pre- `sentedto rectiers 6 and 'I are equal at Fc. If the applied oscillatory energy at winding deviates in frequency from the value Fc in one direction, one rectifier will have a` greater impedance presented thereto than the other rectifier. Hence, n the rectified output of that rectifier will be higher. Should the oscillatory energy shift in frequency to the opposite side of Fc, then the other rectifier will have algreater impedance presented thereto and a correspondingly greater rectified output. The secondary circuit oftransformer I is inthe form' of a seesaw circuit wherein a low impedance at one end of an artificial line is reflected as a high impedance at the other. A yquarter wave section of articial line has this eiiect. The

general characteristics of impedanceY inverting networks. have been described by W. H. Doherty in Proceedings of Institute of Radio Engineers,

September 1936, at page 1172. Consideringthe crystal circuit from this view, when the impedance of crystal P goes low, the impedance at the other endfwhich is fed to the opposite diode, is high.

vacross winding 5, the first step is to short circuit out crystal P and shunt condenser C', and then tune coil 5 by condenser C to the value Fc. second step is to remove the short circuit; the capacity of C must equal that of condenser C. This makes the reactances of coil 5, condenser C and condenser C' all equal.

It'will now be seen that as the applied signal The currents shift in frequency relative to Fc, .the rectiiiers will have the input impedances thereof vary in corresponding complementary fashion.

'This is clearly depicted inFig. 2. Therefore, the

rectifier outputs of the rectiiiers will be unequal, except at Fc. load resistors I2 and I3 are added in polarity opposition, the AFC voltage at the cathode end of resistor I2 will' have a magnitude and polarity dependent on the extent and sense of deviation of the applied signal current frequency relative to Fc. The manner of utilizing this AFC' voltage yfor oscillator frequency correction is well known.

In Fig.f3 there is showna modification of the circuit inFig. ,1, wherein the impedance inversion networkfunctions as the output load of coupling amplifier tubes 2 and 2". The latter tubes have the signal grids thereof in parallel connection to the signal source. The plates of tubes 2' and 2" are connected in push-pull relation to coil 5. The coil 5 in this modification plays the same vpart as windingi of Fig. 1. Rectifier 6 is coupled across the combination of crystal .P and condenser C. The rectifier 'I is coupled across condenser C. 'Ihe junction of resistors I4 and I5 is grounded. The AFCvoltage, taken off from the cathode ends of load resistors I2 and I3, may be fed to any reactancetube device of the push-pull type. The

adjustment of the inversion network will be the same as described for Fig. 1.l The condensers C and C' could be a split-stator condenser. Further, if desired, the crystal P may be shunted by- Y a tuned circuit to provide an adjustment for adjusting the frequency spacing between the peak and dip of the characteristic of the crystal. What is provided in Fig. 3, therefore, is a network 5'- C-C forming a circuit, somewhat like a ,quarter- While I have indicated and described two sysf tems for carryingl my invention into effect, it will be apparent 'to one skilled in the art that my invention is by no means limited to the particular circuitsv shown and described, but that many modifications may be made without departing from'the scope of my'invention as set forth in the appended claims.

What I claim is: f l

l. In a discriminator-rectilier network adapt-ed for automatic frequency control, a pair of opposed rectifiers having a common output Acircuit, a common input network which has a resonant frequency equal-to the mean frequency of applied signal energy, a pair of impedance elements coupled to said input network, one of said impedance elements being a condenser, means for connecting said condenser between the electrodes of one'of the rectiiiers, the second impedance element including a piezo-electric crystal, and means for connecting the tcrystal between the electrodes of the second rectifier.

2. In a discriminator-rectier network adapted for automatic frequency control, a pair of yopposed rectiiiers having va common output circuit,

a common input network which has a resonant frequency equal to the mean frequency of applied signal energy, a pair of impedance elements coupled to said common network, one of said im- Since the rectied voltages across pedance elements having a capacity reactance, means for connecting said one impedance element between the electrodes of one `of the rectiers, the second impedance element being a pieze-electric crystal in shunt with a capacitative reactance, means for connecting the crystal and shunt reactance between the electrodes of the second rectifier, and the impedance elements being so chosen that impedance-frequency characteristics are provided at the input electrodes of said rectiers which are complementary.

3. In a frequency discriminator system, a signal input circuit, an inductance coil coupled thereto, a pair of condensers, of equal capacity, connected in series across said coil, a piezo-electric crystal connected across one condenser, said coil and second condenser being resonant to a predetermined reference frequency in the absence of said crystal and one condenser, and the impedance-frequency characteristics across each of said crystal and said second condenser being complementary.

4. In a discriminator-rectier system, a pair of opposed rectifiers having a common output circuit, an essentially capacitative signal input element connected between the input electrodes of one of said rectiflers, a second signal input element which is frequency discriminatory connected between the input electrodes of the second rectiiier, a common signal input circuit coupled to both of said input elements and said input elem'ents being so relatively chosen that they provide an impedance inversion network which has complementary impedance-frequency characteristics at the respective rectiiier input electrodes.

5. In a discriminator-rectiiier network, a pair of opposed rectifiers having a common output circuit, a common input network which has a resonant frequency equal to the mean frequency of applied signal energy, a pair of impedance elements coupled to said common network, means for connecting one element between the electrodes of one of tbe rectiers, the second irnpedance element being a piezo-electric crystal, and means for connecting the crystal between the electrodes of the second rectifier, said pair of impedance elements being so relatively chosen that they provide an impedance inversion network.

6. In combination, 'a pair of opposed diode rectiiiers having a common resistive output circuit,

a common signal input network for said rectiers which has a resonant frequency equal to the mean frequency `of applied signal energy, a source of signal energy comprising a pair of tubes Ahaving parallel-connected signal input control elements and output elements connected to said common signal input network in push-pull relation, a pair of impedance elements in said common input network, one of 'said impedance elements being a condenser, means for connecting said condenser between the electrodes of one of the rectiiiers, the second impedance element including a piezo-electric crystal, means foriconnecting the crystal between the electrodes of the second rectier.

7. In combination, a signal input circuit, an inductance coil coupled thereto, a pair of condensers connected in series across said coil, a piezo-electric crystal connected vacross one of said condensers, said coil and the other of said condensers being resonant to a predetermined reference frequency in the absence of said crystal and one condenser, the impedance-frequency characteristics across each of said crystal. and said second condenser being complementary, respective rectification devices connected with each of said crystal and second condenser, and

means for deriving from the rectification devices the outputs thereof in polarity opposition.

8. In a frequency discriminator network, a signal input circuit which has a resonant frequency equal to a predetermined reference frequency, a pair of impedance elements being coupled to said input circuit, one of theimpedance elements including a piezo electric crystal, the second impedance element consisting of a condenser, a pair of output terminals connected across said condenser, a second pair of output terminals connected across said crystal, `and said impedance elements being so relatively chosen that complementary impedance-frequency characteristics are provided at the respective pairs of output terminals relative to said reference frequency.

MURRAY G. CROSBY. 

